Test strip qualification system

ABSTRACT

In connection with a fluidic medical diagnostic device that permits measurement of the coagulation time of blood, software, methods and associated devices for quality control are provided. The fluidic device preferably includes a test strip with one end having a sample port for introducing a sample and a bladder at the other end for drawing the sample to a measurement area. A channel carries sample from the sample port to an assay measurement area and first and second control measurement areas. Preferably a stop junction, between the measurement areas and bladder, halts the sample flow for measurement. If results from measurements taken for each control fall within a predetermined zone or defined limits, the assay measurement is qualified. If not, an error is registered and the test strip is counted as unfit.

This is a continuation of U.S. patent application Ser. No. 10/100,254,filed Mar. 14, 2002, now U.S. Pat. No. 6,682,933, issued on Jan. 27,2004.

FIELD OF THE INVENTION

This invention relates to approaches for qualifying results obtained inusing analyte test strips. The invention is particularly suited fortesting the quality of test strips used for measuring prothrombin time(PT time) with whole blood in which a measurement area includes acomposition that catalyzes the blood clotting cascade.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 represent information known in the art and are referenced inthe Background of the Invention. FIG. 4 diagrammatically illustratesaspects of the present invention. Variation of the invention from thatshown in the figures is contemplated.

FIG. 1A is a top view of a test strip as may be used in connection withthe present invention; FIG. 1B is a side view of the test strip.

FIG. 2A is a schematic of hardware elements for a meter for that may beused in the present invention; FIG. 2B shows an alternative variation ofan element of the meter of FIG. 2A.

FIG. 3 is a graph of data that used to determine PT time.

FIG. 4 is a graph showing a qualification zone for an assay secondcontrol.

BACKGROUND OF THE INVENTION

European patent application EP 0 974 840 the (840 publication),published Jan. 26, 2001, describes a device and system that may be usedwith the present invention. FIG. 1 presented herein as adapted from the840 publication shows a parallel multi-channel test strip 2. In it,measurement areas 4, 6 and 8 are provided. Upon introducing a sample,usually whole blood, at introduction port 10 and depressing a bladder 12and releasing it, a partial vacuum draws the blood though channel 14 upto shared stop junction 16. The test strip also includes a bypasschannel 18 which draws sample toward bladder 12 to alleviate negativepressure at the stop junction order to prevent overcoming the surfacetension that pins the fluid in the measurement areas at the stopjunction.

For PT measurements, it is important to stop the flow of sample as itreaches that point to permit reproducible “rouleaux formation”—thestacking of red blood cells—which is an important step in monitoringblood clotting using the present invention. The principle of stopjunction operation is described in U.S. Pat. No. 5,230,866.

A test strip body is described as preferably produced from three layers.The elements above are formed by cutouts in intermediate layer 20,sandwiched between a top layer 22 and bottom layer 24. Preferably, layer22 is double-sided adhesive tape. Stop junction 16 is preferably formedby an additional cutout in layer 22 and/or 24, aligned with the cutoutin layer 22 and sealed with sealing layer 26 and/or 28.

Each cutout for stop junction 16 is preferably at least as wide aschannel 14. A filter may optionally be used to cover sample port 10. Thefilter separates red blood cells from a whole blood sample and/or maycontain a reagent to interact with the blood to provide additionalinformation. A suitable filter comprises an anisotropic membrane,preferably a polysulfone membrane of the type available from SpectralDiagnostics, Inc., (Toronto, Canada). An optional reflector may be on,or adjacent to, a surface or layer of test strip 2 and positioned overthe measurement areas. If a reflector is present, the device becomes atransflectance device.

Typically, in producing the test strip, reagent is bubble-jet printedonto areas 4, 6 and 8. The chemicals at each site are disclosed in the840 publication as: 1) thromboplastin in area 4; 2) thromboplastinbovine eluate, and recombinant Factor VIIa in area 6 and 3)thromboplastin and bovine eluate alone in area 8. The composition inarea 6 is selected to normalize the clotting time-of a blood sample bycounteracting the effect of an anticoagulant, such as warfarin. Thecomposition in area 8 is selected to partially overcome the effect of ananticoagulent. The bovine eluate (plasma barium citrate bovine eluate)is available from Haemotologic Technologies, (Burlington, Vt.);recombinant Factor VIIa from American Diagnostica, (Greenwich, Conn.).Thromboplastin, from Ortho Clinical Diagnostics, (Raritan, N.J.).

After printing, a sample port is cut in untreated polyester film such asAR1235, available from Adhesives Research, (Glen Rock, Pa.) and thenlaminated, in register, to the top of the double-sided tape afterremoving the release layer. A die then cuts the stop junction throughthe three layers of the sandwich. Finally, strips of single-sidedadhesive tape such as MSX4841, available from 3M, (St. Paul, Minn.) areapplied to the outside of the polyester layers to seal the stopjunction.

Use of the test strip can be understood with reference to a schematic ofthe elements of a meter shown in FIGS. 2A and 2B (also adapted from the840 publication), which contemplates an automated meter. Alternatively,manual operation is also possible. In that case, bladder 12 is manuallydepressed before sample is applied to port 10, then released. The firststep the user performs is to turn on the meter, thereby energizing stripdetector 30, sample detector 32, measurement system 34, and optionalheater 36. The second step is to insert the strip. Preferably, the stripis not transparent over at least a part of its area, so that an insertedstrip will block the illumination by LED 38 of detector 40. (Morepreferably, the intermediate layer is formed of a non-transparentmaterial, so that background light does not enter measurement system34.) Detector 40 thereby senses that a strip has been inserted andtriggers bladder actuator 42 to compress bladder 12. A meter display 44then directs the user to apply a sample to sample port 10 as the thirdand last step the user must perform to initiate the measurementsequence. The empty sample port is reflective. When a sample isintroduced into the sample port, it absorbs light from LED 46 andthereby reduces the light that is reflected to detector 48. Thatreduction in light, in turn, signals actuator 42 to release bladder 12.The resultant suction in channel 14 draws sample through the measurementareas to the stop junction. For each measurement area 4, 6 and 8, a LED50 and detector 52 pair is provided to monitor the light transmittedthrough the sample as it is clotting.

Analysis of the transmitted light as a function of time (as describedbelow) permits a calculation of the PT time, which is displayed on themeter display 44 and any messages regarding test strip fitness orreliability. Preferably, sample temperature is maintained at about 37°C. by heater 36. Each such function is controlled by a microprocessorchip 54 controlled by software stored in programmable, read-only memory56.

As described above, the detector senses a sample in sample port 10,simply by detecting a reduction in (specular) reflection of a lightsignal that is emitted by 46 and detected by 48. However, that simplesystem cannot easily distinguish between a whole blood sample and someother liquid (e.g,. blood serum) placed in the sample port in error or,even, an object (e.g., a finger) that can approach sample port 10 andcause the system to erroneously conclude that a proper sample has beenapplied.

To avoid this type of error, another embodiment measures diffusereflection from the sample port. This embodiment appears in FIG. 2B,which shows detector 48 positioned normal to the plane of strip 2. Withthe arrangement shown here, if a whole blood sample has been applied tosample port 10, the signal detected by 48 increases abruptly, because ofscattering in the blood sample, then decreases, because of rouleauxformation. The detector system 32 is thus programmed to require thattype of signal before causing actuator 42 to release bladder 12. Thedelay of several seconds in releasing the bladder does not substantiallyaffect the readings described below.

FIG. 3 depicts a typical “clot signature” curve in which current fromdetector 50 is plotted as a function of time. Blood is first detected ina measurement area at time 1. In the time interval A, between points 1and 2, the blood fills the measurement area. The reduction in currentduring that time interval is due to light scattered by red cells and isthus an approximate measure of the hematocrit. At point 2, sample hasfilled the measurement area and is at rest, its movement having beenstopped by the stop junction. The red cells begin to stack up like coins(rouleaux formation). The rouleaux effect allows increasing lighttransmission through the sample (and less scattering) in the timeinterval between points 2 and 3. At point 3, clot formation endsrouleaux formation and transmission through the sample reaches amaximum. The PT time can be calculated from the interval B betweenpoints 1 and 3 or between 2 and 3. The result is typically reported interms of its “INR” (i.e.,International-Normalized Ratio). Thereafter,the blood changes state from liquid to a semi-solid gel, with acorresponding reduction in light transmission. The reduction in current(C) between the maximum 3 and endpoint 4 correlates with fibrinogen inthe sample.

Measurements made on a whole blood sample using the strip yield a curveof the type shown in FIG. 3 for each of the measurement areas. The datafrom the curves for the controls (measurement areas 6 and 8) are used toqualify the data from the curve for measurement area 4. The measurementof sample from area 4 is validated only when measurements on areas 6 and8 yield results within a predetermined range. If either or both of thesecontrol measurements are outside the range, then a retest with anothertest strip is indicated. Ageing or oxidization of reagents canpotentially yield failing Control 1 an/or Control 2 tests.

SUMMARY OF THE INVENTION

In connection with such a two-control test strip such as described inthe '840 publication, the present invention applies certain criteria toproduce a highly-accurate test strip qualification process based onresults obtained from each control. Results from the first control (C1)are qualified if they fall within a simple numerical range. Whereresults are expressed in terms of INR, C1 readings at or between about0.60 and 1.9 INR are acceptable. Results from the second control (C2)are qualified if they fall within a zone or region bounded by functionsdependent upon assay PT time. When results from the second control andassay are expressed in terms of INR, the functions are line functions,diverging from one another at higher assay INR values. As in thereferenced system, test strip results are qualified, or determined to befit or reliable, upon both C1 and C2 results falling within theprescribed ranges.

Systems of the present invention preferably operate in connection with adisposable test strip and hand held meter as described above.Mathematical algorithms or functions, preferably those described indetail below, are used to qualify test strip data in a highly accuratemanner as evidenced by exemplary results. Such results are preferablyaccomplished with a hand-held meter in a rapid fashion. The algorithmsas implemented by hardware as well as the methodology disclosed formaspects of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In describing the invention in greater detail than provided in theSummary above, the subject test strip qualification system and methodsfor its use are described in relation to FIG. 4 and various equations.Before the present invention is described in such detail, however, it isto be understood that this invention is not limited to particularvariations set forth and may, of courses vary. Various changes may bemade to the invention described and equivalents may be substitutedwithout departing from the true spirit and scope of the invention. Inaddition, many modifications may be made to adapt a particularsituation, material, composition of matter, process, process step orsteps, to the objective, spirit and scope of the present invention. Allsuch modifications are intended to be within the scope of the claimsmade herein.

Furthermore, where a range of values is provided, it is understood thatevery intervening value, between the upper and lower limit of that rangeand any other stated or intervening value in that stated range isencompassed within the invention. The upper and lower limits of thesesmaller ranges may independently be included in the smaller ranges andis also encompassed within the invention, subject to any specificallyexcluded limit in the stated range. Where the stated range includes oneor both of the limits, ranges excluding either both of those includedlimits are also included in the invention. Also, it is contemplated thatany optional feature of the inventive variations described herein may beset forth and claimed independently, or in combination with any one ormore of the features described herein.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present invention, the preferredmethods and materials are described. All existing subject mattermentioned herein (e.g., publications, patents, patent applications aridhardware) is incorporated by reference herein in its entirety. Thereferenced items are provided solely for their disclosure prior to thefiling date of the present application. Nothing herein is to beconstrued as an admission that the present invention is not entitled toantedate such material by virtue of prior invention.

Also, it is noted that as used herein and in the appended claims, thesingular forms “a,” “and,” “said” and “the” include plural referentsunless the context clearly dictates otherwise. Conversely, it iscontemplated that the claims may be so-drafted to require singularelements or exclude any optional element indicated to be so here in thetext or drawings. This statement is intended to serve as antecedentbasis for use of such exclusive terminology as “solely,” “only” and thelike in connection with the recitation of claim elements or the use of a“negative” claim limitation(s).

The present invention is preferably practiced with the test strip andprocedured disclosed in connection with the '840 publication. Accordingto the present invention, C1 includes sufficient coagulation factors tocounteract any effect of anticoagulant—Coumadin in particular—in theblood sample to preferably produce a PT for C1 between 0.8 and 1.9 in agood-quality test strip. C2 differs form C1 both in the composition (asindicated above) and quantity of coagulation factors present in thereagent formulation. Fewer factors are added to C2 to create a partialnormalization of the effects of the anticoagulant in the blood sample.Reaction area 6—corresponding to C1—preferably includes recombinanttissue factor with buffers and preservatives, bovine coagulation factorsof the extrinsic pathway, and recombinant factor VIIa protein. Reactionarea 8—corresponding to C2—preferably includes recombinant tissue factorwith buffers and preservatives, bovine coagulation factors of theextrinsic pathway.

In qualifying test strips, measurements are preferably made on wholeblood sample at each of the three test strip measurement areas, yieldingcurves of the type shown in FIG. 3 used to determine an INR value foreach well. First, whole blood sample is drawn into each of the reactionareas so that the fluid rehydrates the dried reagents and reacts at eachsite. The data obtained for control wells 6 and 8 are used to qualifythe data from the curve from measurement area 4 providing PT time. Thetest results, including that for the controls, is preferably convertedto INR results for use in the algorithms described below and reportingresults to the user.

Based on multiple test strip lots evaluated at multiple clinical sites,it was determined that sufficiently accurate qualification of teststrips results if the PT INR for C1 has an upper limit of about 1.9 anda lower limit of about 0.60 (instead of the more modest range indicatedabove). As for C2 qualification results, is has been observed that C2follows a linear or proportional relationship with the assay resultsobtained. Again, data from multiple test strip lots and clinical siteswas used to generate qualification criteria.

The qualification criteria for C2 may be represented as lines withslightly different slopes and y-intercept values. The slope an upperlimit line 58 as seen in FIG. 4 is greater than that of a lower limitline 60 also seen therein. The lines diverge from one another at higherPT values, thus creating a C2 widening qualification zone 62.

For the second control upper limit 58, when expressed in the formy=m×+b, with m≈0.56 to 0.58 and b≈0.90 the line produce provides anexcellent fit to test data generated. For the second control lower limit60, when expressed in the form y=m×+b, with m≈0.36 and b≈0.37 to 0.38the line produce provides an excellent fit to test data generated. Byuse of the “≈” sign, it is meant equals or is about equal.

In actuality, the line equations described above may been defined withgreater precision. Two significant figures are expressed in order toindicate that variation on such an order is contemplated. (The sameholds true for C1 qualification criteria.) Still, FIG. 4 is drawn withthe precision to which the invention is preferably practiced.

This being said, substantial variability in approach is contemplated aspart of the present invention. For instance, one or more polynomialequations may be used to set the bounds, especially for C2 .Alternately, tabular data representing results within each qualificationranges for C1 and C2, respectively, may be employed. In any event,various qualification zones or regions are defined. Further variationmay include modifying C1 and C2 chemicals. While altering the chemistrymay affect the characteristics of the functions defining thequalification zones, the general nature of the present invention shouldnot change.

In instances where C1 and C2 results are qualified, the test strip meterdisplay 44 shows PT time for the assay (preferably in terms of an INRvalue). If either or both of these control measurements are outside theranges defined, another sort of message indicating test reliability orfitness is displayed by the test strip meter. Error messages specific tothey type of failure may be presented (i.e., messages indicative of C1,C2 or C1 and C2 failure). Alternately, a retest with another test stripmay simply be indicated.

EXAMPLES

A series of trials were conducted in connection, with the presentinvention as early as March 2000. These trials were of an experimentalnature, necessary to determine and/or verify the accuracy of the teststrip qualification approach taught herein. The results of such testinggave positive indication of sufficient accuracy in test strip accuracyachieved through use of the present invention. As of the filing date forthis Specification, no product according to the present invention hasyet been made available to the public.

The accuracy of the inventive methods function was first studied inconnection with subjects enrolled at three independent institutions forevaluation against test strips as described above. In such clinicaltrials, venous blood was drawn and tested on a reference clinicallaboratory device. These results were used for absolute reference. Bycomparison through expanded agreement analysis, which considerscomparison of the clinical interpretation of test results versus theultimate laboratory reference system, inventive system produced a 99%clinical agreement. This rate of agreement evinces significantimprovement over the approach of the CoaguCheck meter by RocheDiagnostics (formerly Boehringer Mannheim Corp.) that produced apublished performance of 87% expanded agreement vs. laboratoryreference. A test strip error frequency of about 0.5% was observed inconnection with these clinical trials for the present invention.

Lay person trials were also conducted on patients who tested themselvesat four defined time intervals in the home environment with the subjecttest strip and meter and were than listed within four hours using venousblood for the reference laboratory system. The home environment/end userresults were compared with the clinical results. This trial alsoproduced an error frequency consistent with the first study at about0.5%. A 0.5% error frequency rate was observed in connection with C1 and0.5% with C2. Errors registered for both C1 and C2 occurred at a 0.1%frequency rate.

In running multiple other tests for verifying test strip accuracy,clinical accuracy of layperson end-users versus the laboratory referencedevice was determined to be in 95% clinical agreement using expandedagreement of result analysis.

Using the cumulative frequency approach, the accuracy of the subjectinvention as preferably practiced in the hands of the layperson end-userin comparison to the laboratory reference can be stated such that 94% ofthe time, the end-use obtains results within 0.5 INR units of thelaboratory reference method. This shows a significant improvement over acorresponding 90% performance rating for results within 0.5 INR aspublished in connection with Avocet Medical products.

Though the invention has been described in reference to a singleexample, optionally incorporating various features, the invention is notto be limited to what is described or indicated as contemplated withrespect to possible variation. The breadth of the present invention isto be limited only by the literal or equitable scope of the followingclaims. That being said,

1. A system for testing the qualification of a test strip for use tomeasure prothrombin (PT) time, said test strip comprising an assayreaction area and a control reaction area, said system comprising: ameter configured for receiving said test strip; and a computer-readablemedium embodying a program to qualify said test strip according to amethod comprising: obtaining PT results for said control reaction area;and comparing results from said control reaction area to controlqualification criteria comprising an upper limit and a lower limit, saidupper limit being about a 1.9 International Normalized Ratio and saidlower limit being about a 0.60 International Normalized Ratio, whereinsaid test strip is qualified if said results fall within said upperlimit and said lower limit.
 2. The system of claim 1, wherein saidProgram further comprises outputting a message to a user indicating teststrip qualification.
 3. A system for testing the qualification of a teststrip for use to measure prothrombin (PT) time, said test stripcomprising an assay reaction area and a control reaction area, saidsystem comprising: a meter configured for receiving said test strip; anda computer-readable medium embodying a program to qualify said teststrip according to a method comprising: obtaining PT results for saidreaction areas; and comparing results from said control reaction area tocontrol qualification criteria comprising an upper limit and a lowerlimit, each being dependent on assay reaction area PT results, whereinsaid test strip is qualified if said results fall within said upperlimit and said lower limit.
 4. The method of claim 3, wherein said upperand lower limits comprise line functions.
 5. The method of claim 4,wherein said line functions are expressed as y=m×+b, wherein yrepresents an International Normalized Ratio results obtained for saidcontrol reaction area, ×represents an International Normalized Ratioresults obtained for said assay reaction area and m≈0.56 to 0.58 andb≈0.90 for said upper limit and wherein m ≈0.36 and b ≈0.37 to 0.38 forsaid lower limit.
 6. The system of claim 5, wherein said program furthercomprises outputting a message to a user indicating test stripqualification.
 7. A system for testing the qualification of a test stripfor use to measure prothrombin (PT) time, said test strip comprising anassay reaction area, a first control reaction area and a second controlreaction area, said system comprising: a meter configured for receivingsaid test strip; and a computer-readable medium embodying a program toqualify said test strip according to a method comprising: obtaining PTresults for each said reaction areas; and comparing results from saidfirst control reaction area to first control qualification criteriacomprising a first upper limit and a first lower limit, said first upperlimit being about a 1.9 International Normalized Ratio and said firstlower limit being about a 0.60 International Normalized Ratio; andcomparing results from said second control reaction area to secondcontrol qualification criteria comprising a second upper limit and asecond lower limit, each being dependent on assay reaction area PTresults; wherein said test strip is qualified if said results from saidfirst control reaction area fall within said first upper limit and saidfirst lower limit and if said results from said second control reactionarea fall within said second upper limit and said second lower limit. 8.The system of claim 7, wherein said program further comprises outputtinga message to a user indicating test strip qualification.